Two AIEEE 2005 Questions on Direct Current Circuits

(1) In the circuit, the galvanometer G shows zero deflection. If the batteries A and B have negligible internal resistance, the value of the resistor R will be

(a) 1000Ω (b) 500Ω (c) 100Ω (d) 200Ω

As there is no current through the galvanometer, the voltage drop across R must be 2V ( so as to balance the emf 2V of the battery B). The voltage drop (produced by the current sent by battery A) across 500Ω is therefore 10V. The same current flows through R and hence R must be 100Ω.

(2) An energy source will supply a constant current in to the load if its internal resistance is

(a) non zero but less than the resistance of the load

(b) zero

(c) very large compared to the load resistance

(d) equal to the resistance of the load

The correct option is (c) since the current will not change with the load resistance if the total resistance of the circuit is almost equal to the source resistance. Ideally, a constant current generator should have infinite resistance.

[You may note also that an ideal constant voltage generator should have zero resistance].